Adverse reactions to food antigens are a common health problem. A principle issue in research into the basis of food allergies has been to understand why some food proteins are allergenic, while others are not. The advent of biotechnology applied to crop production has brought this area of research to greater visibility. To address this question, and on a collaborative project with Dr. P. V. Subba Rao at the Indian Institute of Science and Dr. Brian Martin (NIMH), we have examined proteins in Parthenium hysterophorus, a common weed related to ragweed. The major allergen (designated as Par h 1) is purified using anion exchange chromatography and HPLC. The purified protein is homogenous on SDS-PAGE and reveals a single N-terminal amino acid (lysine) upon Edman degredation. Par h 1 is a glycoprotein with an estimated molecular weight of 31 kd. The carbohydrate moieties appear related to its IgE binding ability. The hydroxyproline-rich region of Par h 1 is 30-40% identical to similar stretches in extensins. IgE antibodies in the sera of parthenium allergic individuals cross-react with a 50 kd hydroxyprolien-arabinose-rich extensin from potato tuber. In summary, plant glycoproteins related to extensins which have carbohydrate- containing IgE binding epitopes, contribute to allergenic cross- reactivity among foods and plant pollens.